Mobile devices, such as cell phones, are generally designed to transmit a radio frequency (RF) signal at a high output power level when located far away from a receiving base station (or when the transmission channel is noisy) to ensure adequate reception of the signal. Conversely, mobile devices are generally designed to transmit a RF signal at a comparatively lower output power level when located close to the receiving base station (or when the transmission channel is less noisy). Mobile devices can determine the power level at which to transmit the RF signal by, for example, using information received from the base station.
A power amplifier is used in a mobile device to amplify the RF signal prior to transmitting it to the base station. Therefore, depending on the distance of the mobile device from the base station (or the conditions of the channel between the mobile device and base station) the gain of the power amplifier can be adjusted to provide more or less amplification of the RF signal prior to transmission. For example, if the distance of the mobile device from the base station is relatively small, the gain of the power amplifier can be reduced to provide less amplification of the RF signal than if the distance of the mobile device from the base station was comparatively larger. Adjusting the gain of the power amplifier depending on the distance of the mobile device from the base station (or conditions of the channel) allows the mobile device to conserve power. In other words, the power amplifier does not need to continually operate under the assumption of worst case operating conditions (e.g., under the assumption that the mobile device is located at a far distance from the base station) and the gain of the power amplifier can be reduced, when operating conditions permit, to conserve power.
However, the power efficiency of a typical power amplifier does not scale linearly with its gain. In general, the typical power amplifier, and the impedance matching network at the output of the power amplifier, are designed to provide maximum power efficiency at a single operating gain. This single operating gain is typically set at the maximum gain that the power amplifier is expected to operate at (or at least at a very high gain). Assuming the power amplifier is designed to operate most efficiently at this high gain setting, any reduction in the gain of the power amplifier will lead to a reduction in the power amplifier's efficiency. Therefore, although reducing the gain of the power amplifier can reduce its power consumption, often the reduction in power consumption is not as significant as desired because of the further reduction in the power amplifier's efficiency.
For example, a power amplifier designed to operate at a high gain level may amplify the power of a RF signal from 1 mW to 500 mW with 40% power efficiency. At 40% power efficiency, the power amplifier will consume around 1.25 W of power. If the gain of the same power amplifier is reduced, however, such that the power of the RF signal is amplified from 1 mW to only 50 mW (a ten times reduction in the gain), the power efficiency of the power amplifier may fall to 5%. At 5% power efficiency, the power amplifier will consume around 1 W. Thus, although the gain of the power amplifier was reduced by a factor of ten, the power consumed by the power amplifier was only reduced by 20%.
Therefore, what is needed is a system and a method for improving the efficiency of power amplifiers used when operating at lower gains or lower output power levels.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.